Reworked constructDataSource.
Reworked constructDataSource.

 * Removed dev_id parameter because the dev_id is in the file name
 * Renamed constructDataSource to a more idiomatic _build_rrd_data_source_definition

StefanoSalsano/OSHI-monitoring,ferrarimarco/OSHI-monitoring,ferrarimarco/OSHI-monitoring,netgroup/OSHI-monitoring,netgroup/OSHI-monitoring,StefanoSalsano/OSHI-monitoring